Dispensers of this kind are already known and have been available to consumers for a long time. The piston that drives the product to be ejected translationally is generally mounted at the end of a threaded rod that cooperates with a nut integrally attached to the dispenser. The rod is in turn driven to rotate incrementally via a pushbutton actuated by the user (see U.S. Pat. No. 4,624,594, for example and U.S. Pat. No. 4,954,000, Granted Sept. 4, 1990.
Filling such a dispenser presents practical problems at the present time. The part of the dispenser including the drive mechanism has a relatively high production cost. To prevent improper handling during filling from making the dispenser unusable and hence unsaleable, it is generally kept separate from the reservoir portion at the moment it is filled. The two parts of the dispenser are then assembled later. It is in this assembly that the difficulty arises, particularly when the piston carried by the rod must be threaded into the tube of the reservoir. When the piston engages the rear end of the reservoir, the layer of air between the end of the product and the face of the piston facing the product is trapped. To expel this layer of air from the reservoir through the spaces existing between the piston and the wall of the reservoir, major effort must be exerted on the piston, with the risk of damaging the dispenser and crushing the product. Moreover, if a layer of air remains between the product and the piston, the action of the piston will necessarily be damped, and its response to manual actions by the user will be altered. Moreover, in the case of a cosmetic product, contact with the trapped air tends to alter the product.